Repeatedly executes the computation p and returns its
results. repeating works best when p is single-valued:
if p is multivalued Orc will spawn a repeating thread for every
result returned, resulting in an exponential blow-up of
threads (XXX: I don't think this was actually intended.)

The rough inverse of sandbox, repeatedly reads values from the
valsMVar until j values have been read or the vals MVar is
exhausted (a Nothing is passed). When there are no more values to
be returned, fills the end MVar.